1. Field of the Invention
This invention relates to micromachined capacitive lateral accelerometer devices and monolithic, three-axis accelerometers having such devices.
2. Background Art
The following references are referred to herein by their reference number:                [1] Yazdi, N., F. Ayazi, and K. Najafi, “Micromachined Inertial Sensors,” PROCEEDINGS OF THE IEEE, 1998. 86 (8 Aug. 1998): p. 1640-1658.        [2] Yazdi, N., “Micro-g Silicon Accelerometers with High Performance CMOS Interface Circuitry, in EECS,” 1999, The University of Michigan: Ann Arbor.        [3] Analog-Devices, ADXL50, Monolithic Accelerometer with Signal Conditioning, Data Sheet. 1996.        [4] Analog-Devices, ADXL105, High Accuracy +−1 g to +−5 g Single Axis iMEMS Accelerometer with Analog Input. 1999.        [5] Lu, C., M. Lemkin, and B. E. Boser, “Monolithic Surface Micromachined Accelerometer with Digital Output,” IEEE JOURNAL OF SOLID-STATE CIRCUITS, 1995, 30 (12 Dec. 1995): p. 1367-1373.        [6] Boser, B. E. and R. T. Howe, “Surface Micromachined Accelerometers,” IEEE JOURNAL OF SOLID-STATE CIRCUITS, 1996. 31 (3 Mar. 1996): p. 366-375.        [7] Cross-bow, High Sensitivity Accelerometer, LF Series Data Sheet, 2001.        [8] Luo, H., G. K. Fedder, and L. R. Carley, “1 mG Lateral CMOS-MEMS Accelerometer,” in 13TH IEEE INTERNATIONAL CONFERENCE ON MICRO ELECTRO MECHANICAL SYSTEMS (MEMS '00). 2000. Miyazaki, Japan.        [9] Wu, J. and L. R. Carley, “A Low-noise Low-offset Chopper-stabilized Capacitive Readout Amplifier for CMOS MEMS Accelerometers, ” in IEEE INTERNATIONAL SOLID STATE CIRCUITS CONFERENCE (ISSCC '02). 2002. San Francisco, Calif.        [10] Lemkin, M., et al., “A Low-noise Digital Accelerometer Using Integrated SOI-MEMS Technology,” in 10TH INTERNATIONAL CONFERENCE SOLID-STATE SENSORS AND ACTUATORS (Transducers '99) 1999. Sendai, Japan.        [11] Ishihara, K., et al., “Inertial Sensor Technology Using DRIE and Wafer Bonding with Interconnecting Capability,” JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, 1999. 8(4 1999): p. 403-408.        [12] Griffin, W., H. Richardson, and S. Yamanami, “A Study of Squeeze Film Damping,” ASME JOURNAL OF BASIC ENGINEERING, 1966: p. 451-456.        [13] Zhang, X. and W. C. Tang, “Viscous Air Damping in Laterally Driven Microresonators,” PROCEEDINGS OF THE IEEE MICRO ELECTRO MECHANICAL SYSTEMS, 1994: p. 199-204.        [14] Henrion, W., et al., “Wide Dynamic Range Direct Digital Accelerometer,” in Solid-State Sensors and Actuators Workshop, 1990, Hilton Head Island, S.C., USA.        [15] Yazdi, N. and K. Najafi, “All-Silicon Single-Wafer Micro-g Accelerometer with a Combined Surface and Bulk Micromachining Process,” JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, 2000. 9(4 Dec. 2000): p. 544-550.        [16] Rudolf, F., et al., “Precision Accelerometers with ug Resolution,” SENSORS AND ACTUATORS, A: Physical, 1990, 21(1-3 2 Pt2 1990): p. 297-302.        [17] Input output, Design Through to Production of a MEMS Digital Accelerometer for Seismic Acquisition, 2001.        [18] Xiao, Z., et al., “Laterally Capacity Sensed Accelerometer Fabricated with the Anodic Bonding and the High Aspect Ratio Etching,” in 10th INTERNATIONAL CONFERENCE SOLID-STATE SENSORS AND ACTUATORS (Transducers '99), 1999, Sendai, Japan.        [19] Chae, J., H. Kulah, and K. Najafi, “A Hybrid Silicon-On-Glass (SOG) Lateral Micro-Accelerometer with CMOS Readout Circuitry,” in 15th IEEE INTERNATIONAL CONFERENCE ON MICRO ELECTRO MECHANICAL SYSTEMS (MEMS '02), 2002, Las Vegas, Nev.        [20] Yazdi, N. and K. Najafi, “All-Silicon Single-Wafer Fabrication Technology for Precision Microaccelerometers,” in 9th INTERNATIONAL CONFERENCE SOLID-STATE SENSORS AND ACTUATORS (Transducers '97), 1997, Chicago, Ill.        [21] Doscher, J., “ADXL105: A Lower Noise, Wider Bandwidth Accelerometer Rivals Performance of More Expensive Sensors,” 1999.        [22] Honeywell, ASA7000, Micromachined Accelerometer, Data Sheet, 2001.        [23] Motorola, MMA2201D, Surface Mount Micromachined Accelerometer, Data Sheet, 2000.        [24] Roylance, L. M. and J. B. Angell, “Batch-Fabricated Silicon Accelerometer, ” IEEE TRANS ELECTRON DEVICES, 1979: p. 1911-1917.        [25] Salian, A., et al., “A High-performance Hybrid CMOS Microaccelerometer,” in SOLID-STATE SENSORS AND ACTUATORS WORKSHOP, 2000, Hilton Head Island, S.C.        [26] Roark, R. J. and W. C. Young, “Roark's Formulas for Stress and Strain,” 6th ed. 1989, New York: McGraw-Hill. xiv, 763.        [27] Warren, K., “Navigation Grade Silicon Accelerometers with Sacrificially Etched SIMOX and BESOI Structure,” in SOLID-STATE SENSORS AND ACTUATORS WORKSHOP, 1994, Hilton Head Island, S.C.        [28] Endevco, “Variable Capacitance Accelerometer, Model 7596, Data Sheet, 2000.        [29] Chae, J., Kulah, H., and Najafi, K., “A Monolithic, 3-Axis Silicon Capacitive Accelerometer with Micro-g Resolution,” in 12th IEEE INTERNATIONAL CONFERENCE ON SOLID-STATE SENSORS, ACTUATORS AND MICROSYSTEMS (Transducers '03), Boston, Mass.        
The following U.S. Pat. Nos. are related to the present invention: 6,035,714; 6,167,757; 6,286,369; and 6,402,968.
The present and potential future markets of microaccelerometers cover a wide range from automotive, biomedical, computer peripheral, and sport equipment applications which require low/medium sensitivity sensors with moderate noise floor to inertial navigation/guidance systems, seismometry, microgravity measurements which demand high sensitivity with very low noise floor. From among a number of sensing methods, the capacitive sensing technique has become the most attractive recently because it provides high sensitivity, low noise performance, good DC response, low temperature sensitivity, and low power dissipation [1, 2].
FIG. 1 shows a conventional capacitive lateral microaccelerometer, generally indicated at 10. A structural mass 11 (proofmass) having comb fingers 12 made of polysilicon is suspended over a silicon substrate 13 such that it is free to move in response to external acceleration. Electrodes 14 are anchored to the substrate 13, and they also have comb fingers 15 (sense fingers) which form parallel plate capacitors with the fingers 12 of the proofmass 11. When an external acceleration is applied to the accelerometer 10, the proofmass 11 moves against the acceleration due to the inertial force while the electrodes 14 remain fixed, which results in capacitance variation between the fingers 12 of the proofmass 11, and the sense fingers 15 on the two sides of the proofmass 11. By measuring the differential capacitance between the capacitors on the two sides of the proofmass 11, the external acceleration is determined.
Most commercialized or developed lateral capacitance microaccelerometers utilize a cantilever-type electrode configuration for force feedback to achieve a higher dynamic range and higher sensitivity [3-5]. The feedback requires stiff electrodes to ensure stability of sensor operation. Thus, it is important for the electrodes to be stiff enough not to bend when the feedback is applied. However, the cantilever-type configuration does not provide adequate stiffness if the proofmass is heavy such that it requires a large force to move, which is the case for a high performance device.
There is a need for a stiffer electrode configuration to overcome the drawback introduced above. Bridge-type electrode configurations as shown in FIG. 3 have been utilized by many Z-axis accelerometers since first Z-axis devices were developed in the early 1980's. It has not been implemented with lateral devices due to relatively small proofmass of the devices and inherent fabrication properties of planar semiconductor process technology. As proofmasses become larger in order to achieve high performance devices, however, a solution to solve the problem is required.
High-sensitivity and low-noise characteristics are some of the most important factors to achieve high performance acceleration sensors because they determine signal-to-noise ratio (SNR). In order to design a high-sensitivity, low-noise capacitive microaccelerometer, one may reduce the sensing gap and damping, and increase mass of a structure and sensing area [2]. A number of microaccelerometers have been reported since the early 1980's with many methods to improve the sensitivity and noise characteristics. Most of them are Z-axis sensors which are sensitive to the vertical direction of acceleration relative to a substrate because it is easier to fabricate a small sensing gap and large-area electrodes with a huge proofmass than in-plane (lateral) devices. There is a need for a large SNR for capacitive micromachined lateral accelerometers.
Conventional surface micromachined lateral accelerometers utilize a thin polysilicon film as their mechanical structure, which is only about 2 μm thick, and define their sensing gap by using RIE technology [3, 4, 6-9]. Since the devices have a very small proofmass, they have suffered from Brownian noise, which is created by random motion of gas molecules, such that it is necessary to increase the mass of the structure in order to achieve high performance devices. Many approaches have been performed to increase the thickness of the structure, while maintaining a small sensing gap and large sense area.
Recently, deep RIE technology has been introduced to provide high-aspect ratio etching profile such that it is combined with a silicon-on-insulator (SOI) wafer or wafer bonding technology to increase the thickness of the structure [10, 11]. However, the ratio of the sensing gap and the height of sense electrodes of the lateral devices is still relatively small (20˜35) comparing with that of Z-axis devices (>100) due to limitations of the deep RIE technology. With defining the sensing gap by RIE or deep RIE technology, it is not likely to attain the high-aspect ratio which provides high sensitivity required for high performance accelerometers.
The damping in micromachined capacitive devices is viscous damping during the movement of a structure, which is categorized into either squeeze film damping or Couette-flow damping [12, 13], as illustrated in FIGS. 2a and 2b, respectively. Although the damping caused by the Couette-flow is much smaller than the squeeze film's, most micromachined capacitive devices are implemented with the squeeze film damping in order to have higher sensitivity. In order to reduce the damping, vacuum packaging or making holes on the parallel plates to let the fluid between two plates escape through the holes have been developed [14, 15]. It is desirable to avoid vacuum packaging if possible, since it is not cost effective. Out-of-plane (Z-axis) devices have implemented the perforation technique to reduce damping significantly [15]. However, from a manufacturing point-of-view, it is not feasible to perforate the plates of in-plane (lateral) devices because the fabrication process of devices has inherent features of a semiconductor technology, so-called planar technology, which is basically to put down and pattern thin films on a substrate.
The most effective way to reduce the squeeze film damping is to increase the sensing gap (d0). On the other hand, increasing the sensing gap compromises sensitivity of devices.
Another method to achieve a high-sensitivity, low-noise device is to have a large proofmass. It is very desirable to have a large proofmass for capacitive devices since both sensitivity and noise characteristics can be improved. To have a large proofmass, a few powerful methods have been developed: thick silicon of SOI wafer; a full wafer thickness of a silicon wafer; and wafer bonding technology [11, 15-17]. Several Z-axis microaccelerometers with large proofmass, at least 500 μm thick, have been developed. On the other hand, only a few lateral devices have been reported which have a large proofmass, which is at most 50 μm thick by implementing the thick silicon of an SOI wafer [10, 11, 18]. A lateral device which has 120 μm thick proofmass has been previously developed using the silicon-on-glass (SOG) structure [19]. However, there is a need for a thicker proofmass in order to achieve high-sensitivity and low-noise devices.
Most commercialized or developed lateral capacitive microaccelerometers utilize force feedback to achieve a higher dynamic range and higher sensitivity [3-6, 8, 10, 17, 20-25]. Feedback force is applied between two sets of comb fingers, one set attached to the substrate and the other set attached to the proofmass. When a voltage is applied between these comb finger sets, electrostatic force is generated which tends to attract the proofmass toward the fixed fingers. Obviously, it is important that the sense fingers be stiff enough so as not to bend when this force is applied. Otherwise, the feedback force introduces non-stable operation [2].
Many lateral devices which have been commercialized or developed using long and narrow comb fingers on a relatively small proofmass (few μ-gram weight, 2˜50 μm thickness). These long and narrow comb fingers cannot be used if the proofmass gets bigger—in the order of milligram weight with full wafer thick (˜500 μm) because the fingers are not stiff enough to resist bending in the direction of the applied force. Therefore, a new configuration of electrodes is needed to provide stable operation for a lateral microaccelerometer with high-sensitivity and low-noise floor.
A Z-axis accelerometer has been used in a bridge-type electrode configuration made of either glass wafers or polysilicon [2, 27]. Large stiffness can be attained when thickness of the electrodes to sense direction becomes large. The Z-axis device utilizes either full wafer thick electrodes or stiffeners in the electrode's design [15, 17, 27]. However, it is not feasible to apply those methods to lateral devices due to inherent characteristics of semiconductor planar fabrication techniques. Conventional cantilever-type accelerometers could improve the stiffness of electrodes by increasing the width of them (note that the width of electrodes is the sense direction in lateral devices). Increasing the width of the electrodes is effective in terms of obtaining large stiffness. On the other hand, it consumes overall device area since electrodes occupy most of the area for conventional devices with a comb finger scheme. Thus, it is not a very advantageous method to just simply increase the width of cantilever electrodes for overall performance improvement.
High-sensitivity and low-noise characteristics are some of the most important factors to achieve high performance acceleration sensors because they determine signal-to-noise ratio (SNR). The SNR indicates directly how small the resolution is that the sensors are able to detect. A number of microaccelerometers have been developed since the early 1980's and some of them are commercialized [3, 4, 7, 21, 22, 28].
Most out-of-plane (Z-axis) microaccelerometers, sensitive to vertical direction of acceleration relative to a substrate, provide a few 10's micro-g resolution. On the other hand, in-plane (lateral) devices have a few 100's micro-g resolution because it has easier-to-implement fabrication technology in order to meet high performance requirements for Z-axis devices. Equation (1) below shows three main parameters determining high-sensitivity, low-noise accelerometers: sensing gap (d0); damping (D); and mass of a structure (M):                     Sensitivity        =                                            Δ              ⁢                                                           ⁢                              C                static                                      a                    =                                                                      ɛ                  o                                ⁢                M                            K                        ⁢                          A                              d                o                2                                                                        (        1        )            (ε0: Permittivity, A: Sense area, K: Spring constant, d0: Sensing gap)   TNEA  =                    4        ⁢                                   ⁢                  K          B                ⁢        TD              M  (TNEA: Total Noise Equivalent Acceleration, KB: Boltzmann Constant, T: Temperature in kelvin, D: Damping, M: Mass of structure)
Although many approaches have been developed in order to improve sensitivity and noise characteristics of lateral accelerometers to be comparable with those of Z-axis devices, high performance lateral devices still require better solutions.
Commercialized or developed capacitive lateral microaccelerometers have used RIE technology to define their sensing gap since it is simple, low-cost and CMOS-fabrication-process compatible. As mentioned earlier, it is necessary to obtain a large sensing area and a small sensing gap for high-sensitivity. The first commercialized capacitive microaccelerometer was developed by Analog Devices, Inc. in 1993. The accelerometer has proofmass and electrodes which are made of 2 μm thick polysilicon from the CMOS process. Due to the small mass of its proofmass, the resolution (noise floor/sensitivity) of the sensor is in the range of mill-g/√{square root over ( )}Hz which does not satisfy the requirements for high performance applications such as inertial navigation, guidance systems, microgravity measurements. Since the devices are fabricated based on surface micromachining technology, the thickness of the polysilicon is limited to ˜5 m with an ˜1.3 μm sensing gap, which provides an aspect ratio of only ˜4 between the sensing gap and height of the electrodes.
Recently, deep RIE technology was introduced to be capable of developing high-aspect ratio structures. Some lateral microaccelerometers took advantage of the technology to obtain high-sensitivity and low-noise characteristics. Nevertheless, the ratio between the sensing gap and the height of electrodes is still small (20˜35) compared with the ratio of Z-axis devices (>100).
Damping is one of the major factors to determine noise floor in microstructure as shown in Equation 1. Most micromachined capacitive devices utilize either squeeze film or Couette-flow damping, as shown in FIGS. 2a and 2b, respectively, as previously mentioned.
Squeeze film damping occurs when two closely spaced plates move toward each other such that the gap distance (d0) changes. When fluid between the plates is squeezed, the fluid moves toward less pressurized space where the squeezed fluid is able to release its pressure, which means fluid tends to move where it is easy to escape from the movement of the plates. Generally, damping is proportional to how difficult the fluid could escape when the two plates move. The gap distance between the two plates is, therefore, the main factor to determine the squeeze film damping. In addition, the squeeze damping is small when one side of the rectangular plates is much larger than the other because it is easy for the fluid to release its pressure by moving along a shorter path toward the larger side, as shown in FIGS. 7a and 7b. In other words, to minimize the squeeze film damping while maintaining the gap distance (and capacitance formed by the two plates), the long rectangular plate design of FIG. 7b takes advantage of the characteristics of the damping to reduce noise floor.
Couette-flow damping, as shown in FIG. 2b, is generated by the motion of a fluid when two plates slide against each other without changing their gap. The main mechanism of the Couette-flow damping is the fluid being dragged by the two plates. Therefore, the gap distance is not a strong function of the damping. With microstructures, the damping caused by the Couette-flow is much smaller compared with the squeeze flow damping introduced above because typically the gap is very small, such as 1˜3 μm.
In order to decrease damping, squeeze damping with a large gap or Couette-flow damping is desirable, which compromises the sensitivity of a capacitive device. Equation (2) shows the sensitivity of the device for two damping mechanisms:                               Sensitivity          ∝                                                                      ɛ                  o                                ⁢                A                                            d                o                2                                      ⁢                                                   ⁢            for            ⁢                                                   ⁢            squeeze            ⁢                                                   ⁢            damping                          ,                                  ⁢                  Sensitivity          ∝                                                                      ɛ                  o                                ⁢                w                                            d                o                                      ⁢                                                   ⁢            for            ⁢                                                   ⁢            Couette            ⁢                          -                        ⁢            flow            ⁢                                                   ⁢            damping                                              (        2        )            
As shown by Equation (2), the squeeze film damping configuration provides higher sensitivity since microstructures have a very small sensing gap (d0). Thus, there is a need to compromise between sensitivity and damping which is a strong parameter to determine noise floor unless a better idea is introduced to avoid tradeoff.
A few approaches to avoid the tradeoff have been presented such as, vacuum packaging and perforation of the moving plate. By vacuum packaging, air molecules between the two plates can be removed such that much less fluid motion is generated when the two plates move toward each other. The vacuum packaging is the ultimate method to reduce the damping although it is expensive. On the other hand, perforation of the moving plate would be a smart way to avoid squeeze film damping. A Z-axis device has implemented this technique, which provides more than 100 times reduction in the damping [20]. However, it is not feasible to perforate the plate of lateral devices because the fabrication procedure has inherent characteristics of a semiconductor planar technology, which is basically to deposit and pattern films on a flat substrate.